This invention relates to a multilayer metal-coated steel sheet having a good corrosion resistance, heat resistance and formability. More particularly, it relates to a multilayer metal-coated steel sheet provided on its surface with a Zr coating layer on which only an Al coating layer is formed or an Al-Zr alloy coating layer containing 1 to 50% by weight of Zr and an Al coating layer are formed.
Al-coated steel sheets have been hitherto used in a large quantity. Those which are in common use among them are hot dip Al-coated steel sheets comprising a single coating layer formed by directly coating Al on a steel sheet by hot dipping.
Such hot dip Al-coated steel sheets can achieve a good corrosion resistance and heat resistance so long as they have a coating layer with a thickness of 8 .mu.m or more, and hence have been in wide use for exhaust gas system members of automobiles, combustion machinery system members, household equipment members, etc. In some uses, however, there is recently a demand for thin-coating steel sheets for the purpose of reducing cost for the manufacture of members, or studies are made on whether their uses can be expanded to a use in which steel sheets may be affected by corrosion due to salt damage from sea water.
Where, however, the hot dip Al-coated steel sheets are used in an environment that may cause the corrosion due to salt damage, e.g., an environment in which halogen ions are present, an oxide film formed on the surface, having a corrosion resistance, is solved to produce thereon white rust mainly composed of Al(OH).sub.3, or to cause pit corrosion reaching the substrate steel.
Now, when they may be affected by salt damage in their use, the coating layer is made to have a thickness of as large as 40 .mu.m so that the corrosion does not reach the substrate steel in a short time. There, however, has been the problem that making the coating layer have such a large thickness results in a lowering of formability and also a high cost. The coating surface may also be subjected to chemical conversion such as chromate treatment that is a post treatment commonly applied on coated steel sheets. This treatment, however, can only bring about a temporary effect, and can not fundamentally improve the corrosion resistance.
In the hot dip Al-coated steel sheets, Si is incorporated in the coating layer usually in an amount of 8 to 12% so that an Al-Fe alloy layer formed at the interface between the substrate steel and the coating layer and having a poor formability can be prevented from being developed. In such an instance, if the substrate steel is a low-carbon rimmed steel or Al killed steel usually used, the interface between the coating layer and the substrate steel may be preferentially oxidized when heated to 600.degree. C. or above, and the coating layer may be made into an alloy when heated at 700.degree. C. or above to cause peeling. Thus the incorporation of Si is not necessarily satisfactory in view of corrosion resistance. This problem on corrosion resistance can be settled by use of a substrate steel comprising a low carbon, Ti-added steel or very low carbon, 0.2Ti-added steel that is durable to repeated heating at 650.degree. C. or above. Use of such a Ti-added steel may undesirably result in a relatively high manufacture cost, compared with the Al killed steel or the like. Even with use of the substrate steel comprising the Ti-added steel, a brittle Al-Fe-Si alloy layer is produced at the interface between the substrate steel and the coating layer, and hence no satisfactory formability can be obtained.
On the other hand, an Al-coated steel sheet in which no Al-Fe alloy coating layer may be produced at the interface between the substrate steel and the coating layer can be manufactured if coating is carried out by Al electrodeposition or Al vacuum deposition. However, Al-coated steel sheets manufactured by such processes have a poor heat resistance because the heating at a high temperature causes diffusion of Al of the coating layer into the substrate steel to result in oxidation of the substrate steel. Pinholes reaching the substrate steel are also present in the coating layer, and hence there is a problem on the corrosion resistance in a severe corrosive environment